Category Archives: Quantum Physics

July 4, 2017 by Lea Kivivali Credit: Swinburne University of Technology

By gently prodding a swirling cloud of supercooled lithium atoms with a pair of lasers, and observing the atoms' response, researchers at Swinburne have developed a new way to probe the properties of quantum materials.

Quantum materialsa family that includes superfluids, superconductors, exotic magnets, ultracold atoms and recently-discovered 'topological insulators'display on a large scale some of the remarkable quantum effects usually associated with microscopic and subatomic particles.

But, while quantum mechanics explains the behaviour of microscopic particles, applying quantum theory to larger systems is far more challenging.

"While the potential of quantum materials, such as superconductors, is undeniable, we need to fully grasp the underlying quantum physics at play in these systems to establish their true capabilities," says Chris Vale, an Associate Professor at the Centre for Quantum and Optical Science, who led the research. "That's a big part of the motivation for what we do."

Associate Professor Vale and his colleagues, including Sascha Hoinka and Paul Dyke, also at Swinburne, developed a new way to explore the behaviour of this family of materials. They detected when a 'Fermi gas' of lithium atoms, a simple quantum material, entered a quantum 'superfluid' state.

New system checks theories against experiment

Their system allows theories of superconductivity and related quantum effects to be precisely checked against experiment, to see whether the theories are accurate and how they could be refined.

The researchers' advance was based on the fact that quantum materials' special properties emerge when their constituent particles enter a synchronised state. The zero-resistance flow of electrons through superconductors, for example, emerges when electrons can team up to form 'Cooper pairs'.

The team's sophisticated experimental set-up allowed this co-ordinated quantum behaviour to be detected. By fine-tuning the interaction of their lasers with the Fermi gas, Associate Professor Vale and his colleagues were for the first time able to detect the elusive, low energy Goldstone mode, an excitation that only appears in systems that have entered a synchronised quantum state.

"Because our experiment provides a well-controlled environment and the appearance of the Goldstone mode is very clear, our measurements provide a benchmark that quantum theories can be tested against before they're applied to more complex systems like superconductors," Associate Professor Vale says.

"By developing methods to understand large systems that behave quantum mechanically, we're building the knowledge base that will underpin future quantum-enabled technologies."

The team's research has been published in the online journal Nature Physics.

Explore further: Frequency modulation accelerates the research of quantum technologies

More information: Sascha Hoinka et al. Goldstone mode and pair-breaking excitations in atomic Fermi superfluids, Nature Physics (2017). DOI: 10.1038/nphys4187

Many modern technological advances and devices are based on understanding quantum mechanics. Compared to semiconductors, hard disk drives or lasers, quantum devices are different in the sense that they directly harness quantum ...

Quantum field theories are often hard to verify in experiments. Now, there is a new way of putting them to the test. Scientists have created a quantum system consisting of thousands of ultra cold atoms. By keeping them in ...

Researchers have discovered half-quantum vortices in superfluid helium. This vortex is a topological defect, exhibited in superfluids and superconductors, which carries a fixed amount of circulating current. These objects ...

Work of physicists at the University of Geneva (UNIGE), Switzerland, and the Swiss Federal Institute of Technology in Zurich (ETH Zurich), in which they connected two materials with unusual quantum-mechanical properties through ...

Using some of the largest supercomputers available, physics researchers from the University of Illinois at Urbana-Champaign have produced one of the largest simulations ever to help explain one of physics most daunting problems.

In experiments with magnetic atoms conducted at extremely low temperatures, scientists have demonstrated a unique phase of matter: the atoms form a new type of quantum liquid or quantum droplet state. These so called quantum ...

Researchers at the University of Illinois at Urbana-Champaign and Princeton University have theoretically predicted a new class of insulating phases of matter in crystalline materials, pinpointed where they might be found ...

The last time you watched a spider drop from the ceiling on a line of silk, it likely descended gracefully on its dragline instead of spiraling uncontrollably, because spider silk has an unusual ability to resist twisting ...

During their research for a new paper on quantum computing, HongWen Jiang, a UCLA professor of physics, and Joshua Schoenfield, a graduate student in his lab, ran into a recurring problem: They were so excited about the progress ...

Scientists at the Department of Energy's SLAC National Accelerator Laboratory and Stanford University have made the first direct measurements, and by far the most precise ones, of how electrons move in sync with atomic vibrations ...

A team of scientists has found evidence for a new type of electron pairing that may broaden the search for new high-temperature superconductors. The findings, described in the journal Science, provide the basis for a unifying ...

Solitary waves known as solitons appear in many forms. Perhaps the most recognizable is the tsunami, which forms following a disruption on the ocean floor and can travel, unabated, at high speeds for hundreds of miles.

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Supercool breakthrough brings new quantum benchmark - Phys.org - Phys.Org

Based in Detroit, Michigan, Americas capital for electric-vehicle manufacturing, Electric & Hybrid Vehicle Technology Expo highlights advances right across the powertrain. From passenger and commercial vehicles to off-highway industrial vehicles, this manufacturing and engineering event showcases the latest innovations across a vast range of vehicles. Running concurrent to the exhibition is the Electric & Hybrid Vehicle Technology Conference, which attracts technical leaders and executives from global technology companies to reveal what is driving demand, and shaping novel technologies and new innovations at the cutting edge.

The wide-ranging sessions cover performance vehicle technology transfer, technology transfer from aerospace to EV, technologies for improving efficiency and performance of H/EVs, the impact of autonomous driving features, 48V and low-voltage mild-hybrid architectures (including energy storage design considerations), electric and hybrid bus development, the commercial and vocational electric vehicle sector, P0-P4 architectures and more.

Since 2010 this dual event has experienced exponential growth achieving a sell-out exhibition and record attendance year on year, and bringing in some of the leading names as exhibitors, speakers, delegates and visitors, including Mercedes-Benz, Toyota, American Airlines, Hyundai, Ford, Valeo, BorgWarner, NovaBus, Chrysler, NASA, GM and many more.

Electric & Hybrid Vehicle Technology Expo is attended by industry leaders, businesspeople, technicians, consultants, and research and development professionals, all looking for greater efficiency, safety, and cost reduction.

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Telecommunications, Meet Quantum Physics - Electronics360

There are many popular memes on the Internet that have to do with differing perceptions.

They have multiple photos captioned: What I think I do; What my friends think I do; What my mother thinks I do; and, finally, What I really do.

The pictures usually show wildly differing perceptions of the same job. This also appears to be the case with science. There is often a vast gulf between what people think about science and what it truly is.

Most people tend to think of science as the queen of the intellectual disciplines - always sure and precise, having all the answers to any conceivable question. Sadly, nothing could be further from the truth. Even science itself recognized this as truth with the division between theoretical and practical sciences.

If science is the queen of intellectual disciplines, Physics is the king of science. It is the fundamental investigation into how the world around us works. It includes chemistry, biology, mechanics and just about anything else you can think of. Physics stands astride of science like a Colossus, proud, sure and confident. But this is only a faade. There is a fundamental contradiction inside physics that has defied explanation for the past 100 years. And we are, even now, only beginning to glimpse some faint ideas about how this contradiction can be resolved.

In physics, there are two theories that form the basis for our understanding of the universe. Quantum physics that has explained how matter is constructed and why it behaves the way that it does. Most nuclear physics deals almost exclusively with quantum physics.

On the other end of the spectrum of physics knowledge is relativity. One man, Albert Einstein, whose very name has become another way of saying genius, was responsible for this wonderful theory that is master of everything large. It deals with the structure of the universe, the nature of gravity and explains space and time. It is a theory that has stood every test that has been put to it and it has never failed to produce the expected results or even a slight deviation from the expected results.

Both quantum theory and relativity are two of the most successful theories that we have ever had. The problem is that they don't play well together. That's right, two theories that are as close to reality as we have ever come are not compatible with each other. Doesn't make sense, does it? When you try to apply relativity to the very small scales of the atomic realm, suddenly the mathematics does not make sense any more. Quantities become infinite and predictions go wildly astray.

How is this possible?

If I could answer that question, I would be preparing my speech for my Nobel Prize ceremony. The thing that makes this amazing is that each theory is so close to describing reality that it is almost inconceivable that it could be incorrect. If either or, indeed, both theories are wrong, it will bring about a complete revolution in our understanding of reality.

Some years ago, I visited CERN in Geneva just a couple of months before its discovery of the Higgs particle that controls the mass of matter. CERN is the world's largest scientific apparatus and is designed to smash atoms together at almost the speed of light and then analyze the pieces to understand how matter works. I managed to have lunch in the cafeteria there with some of the scientists working on this marvellous machine. Sitting not too far away were at least two Nobel Prize winners who were doing work at CERN.

The conversation took an interesting turn when I asked them what would happen if the machine did not find evidence of the Higgs particle. The fellow I was talking to got a faraway look in his eyes and said; "Then physics would become very interesting. Something unexpected means that we don't understand it all and we would have to become very creative to figure out what is going on because everything else fits our current theories."

Nobel laureate Richard Feynman agreed with this assessment when he said that physics required a great deal of imagination, but imagination in a straitjacket. This means that you cannot imagine anything you like. What you theorize must also conform to everything we already know. In other words, any new theory must not only explain the new phenomena, but must still provide an explanation for all the old phenomena as well, or, at the very least, not be incompatible with what we observe.

This is the situation for modern physics. We have two incredibly detailed and effective theories of how various parts of nature work, and they are not compatible.

It would seem that physics is indeed "interesting again."

Tim Philp has enjoyed science since he was old enough to read. Having worked in technical fields all his life, he shares his love of science with readers weekly. He can be reached by e-mail at: tphilp@bfree.on.ca or via snail mail c/o The Expositor.

Brantford Expositor 2017

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Why can't quantum theory and relativity get along? - Brantford Expositor

June 29, 2017 by Ali Sundermier A false color image of one of the researchers' samples. Credit: University of Pennsylvania

Researchers from the University of Pennsylvania, in collaboration with Johns Hopkins University and Goucher College, have discovered a new topological material which may enable fault-tolerant quantum computing. It is a form of computing that taps into the power of atoms and subatomic phenomena to perform calculations significantly faster than current computers and could potentially lead to advances in drug development and other complex systems.

The research, published in ACS Nano, was led by Jerome Mlack, a postdoctoral researcher in the Department of Physics & Astronomy in Penn's School of Arts & Sciences, and his mentors Nina Markovic, now an associate professor at Goucher, and Marija Drndic, Fay R. and Eugene L. Langberg Professor of Physics at Penn. Penn grad students Gopinath Danda and Sarah Friedensen, who received an NSF fellowship for this work, and Johns Hopkins Associate Research Professor Natalia Drichko and postdoc Atikur Rahman, now an assistant professor at the Indian Institute of Science Education and Research, Pune, also contributed to the study.

The research began while Mlack was a Ph.D. candidate at Johns Hopkins. He and other researchers were working on growing and making devices out of topological insulators, a type of material that doesn't conduct current through the bulk of the material but can carry current along its surface.

As the researchers were working with these materials, one of their devices blew up, similar to what would happen with a short circuit.

"It kind of melted a little bit," Mlack said, "and what we found is that, if we measured the resistance of this melted region of one of these devices, it became superconducting. Then, when we went back and looked at what happened to the material and tried to find out what elements were in there, we only saw bismuth selenide and palladium."

When superconducting materials are cooled, they can carry a current with zero electrical resistance without losing any energy.

Topological insulators with superconducting properties have been predicted to have great potential for creating a fault-tolerant quantum computer. However, it is difficult to make good electrical contact between the topological insulator and superconductor and to scale such devices for manufacture, using current techniques. If this new material could be recreated, it could potentially overcome both of these difficulties.

In standard computing, the smallest unit of data that makes up the computer and stores information, the binary digit, or bit, can have a value of either 0, for off, or 1, for on. Quantum computing takes advantage of a phenomenon called superposition, which means that the bits, in this case called qubits, can be 0 and 1 at the same time.

A famous way of illustrating this phenomenon is a thought experiment called Schrodinger's cat. In this thought experiment, there is a cat in a box, but one doesn't know if the cat is dead or alive until the box is opened. Before the box is opened, the cat can be considered both alive and dead, existing in two states at once, but, immediately upon opening the box, the cat's state, or in the case of qubits, the system's configuration, collapses into one: the cat is either alive or dead and the qubit is either 0 or 1.

"The idea is to encode information using these quantum states," Markovic said, "but in order to use it in needs to be encoded and exist long enough for you to read."

One of the major problems in the field of quantum computing is that the qubits are not very stable and it's very easy to destroy the quantum states. These topological materials provide a way of making these states live long enough for to read them off and do something with them, Markovic said.

"It's kind of like if the box in Schrodinger's cat were on the top of a flag pole and the slightest wind could just knock it off," Mlack said. "The idea is that these topological materials at least widen the diameter of the flag pole so the box is sitting on more a column than a flag pole. You can knock it off eventually, but it's otherwise very hard to break the box and find out what happened to the cat."

Although their initial discovery of this material was an accident, they were able to come up with a process to recreate it in a controlled way.

Markovic, who was Mlack's advisor at Johns Hopkins at the time, suggested that, in order to recreate it without having to continually blow up devices, they could thermally anneal it, a process in which they put it into a furnace and heat it to a certain temperature.

Using this method, the researchers wrote, "the metal directly enters the nanostructure, providing good electrical contact and can be easily patterned into the nanostructure using standard lithography, allowing for easy scalability of custom superconducting circuits in a topological insulator."

Although researchers already have the capability of making a superconducting topological material, there's a huge problem in the fact that, when they put two materials together, there's a crack in between, which decreases the electrical contact. This ruins the measurements that they can make as well as the physical phenomena that could lead to making devices that will allow for quantum computing.

By patterning it directly into the crystal, the superconductor is embedded, and there are none of these contact problems. The resistance is very low, and they can pattern devices for quantum computing in one single crystal.

To test the material's superconducting properties, they put it in two extremely cold refrigerators, one of which cools down to nearly absolute zero. They also swept a magnetic field across it, which would kill the superconductivity and the topological nature of the material, to find out the limitations of the material. They also did standard electrical measurements, running a current through and looking at the voltage that is created.

"I think what is also nice in this paper is the combination of the electrical transport performance and the direct insights from the actual device materials characterization," Drndic said. "We have good insights on the composition of these devices to support all these claims because we did elemental analysis to understand how these two materials join."

One of the benefits of the researchers' device is that it's potentially scalable, capable of fitting onto a chip similar to the ones currently in our computers.

"Right now the main advances in quantum computing involve very complicated lithography methods," Drndic said. "People are doing it with nanowires which are connected to these circuits. If you have single nanowires that are very, very tiny and then you have to put them in particular places, it's very difficult. Most of the people who are on the forefront of this research have multimillion-dollar facilities and lots of people behind them. But this, in principle, we can do in one lab. It allows for making these devices in a simple way. You can just go and write your device any way you want it to be."

According to Mlack, though there is still a fair amount of limitation on it; there's an entire field that has sprouted up devoted to coming up with new and interesting ways to try to leverage these quantum states and quantum information. If successful, quantum computing will allow for a number of things.

"It will allow for much faster decryption and encryption of information," he said, "which is why some of the big defense contractors in the NSA, as well as companies like Microsoft, are interested in it. It will also allow us to model quantum systems in a reasonable amount of time and is capable of doing certain calculations and simulations faster than one would typically be able to do."

It's particularly good for completely different kinds of problems, such as problems that require massive parallel computations, Markovic said. If you need to do lots of things at once, quantum computing speeds things up tremendously.

"There are problems right now that would take the age of the universe to compute," she said.

"With quantum computing, you'd be able to do it in minutes." This could potentially also lead to advances in drug development and other complex systems, as well as enable new technologies.

The researchers hope to start building some more advanced devices that are geared towards actually building a qubit out of the systems that they have, as well as trying out different metals to see if they can change the properties of the material.

"It really is a new potential way of fabricating these devices that no one has done before," Mlack said. "In general, when people make some of these materials by combining this topological material and superconductivity, it is a bulk crystal, so you don't really control where everything is. Here we can actually customize the pattern that we're making into the material itself. That's the most exciting part, especially when we start talking about adding in different types of metals that give it different characteristics, whether those be ferromagnetic materials or elements that might make it more insulating. We still have to see if it works, but there's a potential for creating these interesting customized circuits directly into the material."

Explore further: Group works toward devising topological superconductor

More information: Jerome T. Mlack et al, Patterning Superconductivity in a Topological Insulator, ACS Nano (2017). DOI: 10.1021/acsnano.7b01549

The experimental realization of ultrathin graphene - which earned two scientists from Cambridge the Nobel Prize in physics in 2010 - has ushered in a new age in materials research.

The 'quantized magneto-electric effect' has been demonstrated for the first time in topological insulators at TU Wien, which is set to open up new and highly accurate methods of measurement.

University of Pennsylvania researchers are now among the first to produce a single, three-atom-thick layer of a unique two-dimensional material called tungsten ditelluride. Their findings have been published in 2-D Materials.

The global race towards a functioning quantum computer is on. With future quantum computers, we will be able to solve previously impossible problems and develop, for example, complex medicines, fertilizers, or artificial ...

Researchers have shown how to create a rechargeable "spin battery" made out of materials called topological insulators, a step toward building new spintronic devices and quantum computers.

In an article published today in the journal Nature, physicists report the first ever observation of heat conductance in a material containing anyons, quantum quasiparticles that exist in two-dimensional systems.

Researchers from the University of Pennsylvania, in collaboration with Johns Hopkins University and Goucher College, have discovered a new topological material which may enable fault-tolerant quantum computing. It is a form ...

By measuring the random jiggling motion of electrons in a resistor, researchers at the National Institute of Standards and Technology (NIST) have contributed to accurate new measurements of the Boltzmann constant, a fundamental ...

Van der Waals interactions between molecules are among the most important forces in biology, physics, and chemistry, as they determine the properties and physical behavior of many materials. For a long time, it was considered ...

For surfers, finding the "sweet spot," the most powerful part of the wave, is part of the thrill and the challenge.

With leading corporations now investing in highly expensive and complex infrastructures to unleash the power of quantum technologies, INRS researchers have achieved a breakthrough in a light-weight photonic system created ...

The technological future of everything from cars and jet engines to oil rigs, along with the gadgets, appliances and public utilities comprising the internet of things, will depend on microscopic sensors.

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New method could enable more stable and scalable quantum computing, physicists report - Phys.Org

In nature, change is constant and inevitable. It is also fairly slow and mainly evolutionary. At the macro level, everything looks very logical, guided by the basic scientific laws of physics, chemistry and biology. We are comfortable with changes that we can observe and measure. We feel that we are in full control of predicting future movements, through elegant mathematical modeling and good enough approximations. The world of macro physics is full of order that is guided by clear scientific standards.

By digging deeper into the area of subatomic particles and quantum physics, things start to look blurry, counter-intuitive and completely unexpected. Old silos of physics, chemistry and biology, as distinct scientific disciplines, start to disappear. We cant clearly observe and freely measure any process, without danger of ruining and completely skewing the results of the very same measurement. We feel amazed and fascinated by the apparent chaos, but also confused and often scared by our inability to comprehend and predict whats next. Thats the domain reserved only for the fearless and most curious minds. Imagination and intuition rule this world, without clear standards and without obvious order.

The physics reality of payments

In the world of payments, I see similar patterns. The traditional payments are ruled by established standards and are protected by clear rules, regulations and relatively high barriers to entry. These sometimes rigid Newtonian laws of payments industry were established over several decades by payment networks like Visa, Mastercard, etc. Traditional FIs feel very comfortable here since they know how to play by the rules and they excel at it. Thats why we enjoy pretty good safety and security of in-store payments today. The standards like EMV, ISO 8583, ISO 20022, PCI DSS are just some of the examples illustrating the point. However, today some of these standards (not all) start to feel old and somewhat inefficient in dealing with some of the demands of the modern payments trends.

On the other hand, in the payments innovation space, we feel like operating inside the subatomic world and space of the payment industry. Similar to the world of quantum physics, frequently, there are no clear rules, and imagination and intuition are often required to be relied on in order to invent and launch new services and products. Disruption of the old business models is ultimately at stake. The new business models are often not easily understood by payment traditionalists. As such, the payments innovation space is opportunistic and exciting, wide open for creative players, but at the same time, it is full of risks for potential investors and incumbents, which are faced with the inability to clearly distinguish winners from losers early enough.

Take online payments as an example there is no clear standard here. It represented the Wild West of the payments industry in the last couple of decades. It is a space that is still filled with significant security risks and friction. Agile and nimble FinTechs may thrive in such an environment, feeling free to experiment, unbound by any of the regulations and unconstrained by a traditionalist mindset. No wonder that incumbent FIs together with Visa and Mastercard have been somewhat marginal players here, despite their ability to rule the world of physical POS payment rails for over half a century now.

Blockchain is an even better example of the financial industrys quantum world. It feels directionless, void of any clear standards and rules, combined with quirky and muddy explanations of underlying consensus-reaching algorithms. It is a fertile ground for buzzwords and skilled snake oil type salesmen, further amplifying the inherent sense of confusion and unpredictability. Despite all of the hype and attention, however, blockchains disruptive potential has not been realized in real life so far. Key questions still galore: which blockchain platform to choose? Are the empirics behind the various consensus recipes trustworthy enough and mathematically provable? How do we deal with inherent scalability challenges for real-time payments? The quest for suitable use cases still continues, but it is starting to feel like we are quickly approaching the point where the whole blockchain movement may need to detach itself from the original (traditionalist) route and creatively explore some of the unusual paths and back roads, to be able to deliver promised breakthrough innovations.

It should be obvious by now, that the two worlds of payments traditional and innovation are not compatible. How do we move forward then?

What can be done?

Lets go back to the physics field for potential inspiration and guidance. Physicists clearly recognized the chasm and impedance mismatch between traditional and quantum science and are patiently working together to bridge the incompatible views. The relentless pursuit of the (still elusive) theory of everything in physics is underway, with many colliding theories in existence, but with everybody marching toward the same important goal here. Physicists on all sides of the scientific spectrum clearly understand the need for healthy open-minded collaboration toward final convergence and harmonization of all of their existing incompatible views. Although it may not be obvious, they are in my opinion perfect example of agile innovators, not afraid to try any promising theory, challenge it and pivot if required or adopt and build on it. They are also brutal realists, well aware that their goal of ultimate convergence can only be enabled by solid standardization along the way.

Now, back to payments again. The good news is that standardization in the payments space is not limited, in any way, by our ability to understand unpredictable laws of the subatomic world, but purely by the willingness of all involved players to systematically collaborate and create necessary standards that enable progress. Nimble and agile FinTechs may feel they are more adept to play in chaotic innovation space, but it is in their best interest to realize as soon as possible that they shall enable their offerings for easy integration with the incumbents, in order to be seriously considered as future partners. Incumbents, on the other hand, must realize that they cant keep protecting their current business models forever, and shall become open-minded toward emerging payment innovations.

In online payments, for example, the upcoming W3C Payment Request and W3C Payment App standard APIs will enable direct communication between online merchants and the providers of online payment app browser plug-ins. Will both merchants and FIs recognize the potential of this standard and seize this opportunity? It can clearly give innovative FIs a chance to painlessly establish themselves as natural online payment providers for their current customers. It also enables merchants to integrate only with 1 standard API for initiation of online payments and thus eliminate the need for multiple Pay With buttons on their checkout pages, each involving costly integration with a different set of APIs today. This is a huge opportunity and a clear candidate for theory of everything in the field of online payments. The process of online payment space standardization may likely expose PayPal as obsolete and unnecessary, after several decades of ruling the same space. Since this clearly benefits online merchants and FIs, I hope they will start collaborating intensely in 2017.

In the blockchain space, FinTechs must recognize that lack of standards, lack of clarity on the underlying consensus mechanisms and lack of scalability for real-time payments seriously impedes the adoption of their incompatible platforms. In my opinion, the set of common industry-standard APIs for blockchain is long overdue and initiating work must be the next biggest priority for the blockchain community in 2017. Why not again use W3C as a natural and neutral facilitator for this standardization? One day, the FIs should ultimately be able to experiment efficiently by plugging in blockchain platform A, then plug in blockchain platform B, in order to evaluate and compare, without the need to completely rewrite their application code. Further, the required scalability for real-time payments is hard to deliver elegantly using any of the current blockchain platforms. Here, openness to new ideas which might be radically different than the current mainstream thinking is clearly needed.

Will future deliver tangible solutions for some of these challenges? No crystal ball here, but I personally feel pumped up and am enthusiastically looking forward to our collective quest for the much needed theory of everything and standardization for every amazing sub-field of payments.

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Payments Innovation - A Quantum World Of Payments - Finextra (blog)

Although the term quantum computer might suggest a miniature, sleek device, the latest incarnations are a far cry from anything available in the Apple Store. In a laboratory just 60 kilometers north of New York City, scientists are running a fledgling quantum computer through its paces and the whole package looks like something that might be found in a dark corner of a basement. The cooling system that envelops the computer is about the size and shape of a household water heater.

Beneath that clunky exterior sits the heart of the computer, the quantum processor, a tiny, precisely engineered chip about a centimeter on each side. Chilled to temperatures just above absolute zero, the computer made by IBM and housed at the companys Thomas J. Watson Research Center in Yorktown Heights, N.Y. comprises 16 quantum bits, or qubits, enough for only simple calculations.

If this computer can be scaled up, though, it could transcend current limits of computation. Computers based on the physics of the supersmall can solve puzzles no other computer can at least in theory because quantum entities behave unlike anything in a larger realm.

Quantum computers arent putting standard computers to shame just yet. The most advanced computers are working with fewer than two dozen qubits. But teams from industry and academia are working on expanding their own versions of quantum computers to 50 or 100 qubits, enough to perform certain calculations that the most powerful supercomputers cant pull off.

The race is on to reach that milestone, known as quantum supremacy. Scientists should meet this goal within a couple of years, says quantum physicist David Schuster of the University of Chicago. Theres no reason that I see that it wont work.

Cooling systems (Googles shown) maintain frigid temperatures for the superconducting quantum processor, which sits at the bottom of the contraption. The system is enclosed in a water heatersized container.

But supremacy is only an initial step, a symbolic marker akin to sticking a flagpole into the ground of an unexplored landscape. The first tasks where quantum computers prevail will be contrived problems set up to be difficult for a standard computer but easy for a quantum one. Eventually, the hope is, the computers will become prized tools of scientists and businesses.

Some of the first useful problems quantum computers will probably tackle will be to simulate small molecules or chemical reactions. From there, the computers could go on to speed the search for new drugs or kick-start the development of energy-saving catalysts to accelerate chemical reactions. To find the best material for a particular job, quantum computers could search through millions of possibilities to pinpoint the ideal choice, for example, ultrastrong polymers for use in airplane wings. Advertisers could use a quantum algorithm to improve their product recommendations dishing out an ad for that new cell phone just when youre on the verge of purchasing one.

Quantum computers could provide a boost to machine learning, too, allowing for nearly flawless handwriting recognition or helping self-driving cars assess the flood of data pouring in from their sensors to swerve away from a child running into the street. And scientists might use quantum computers to explore exotic realms of physics, simulating what might happen deep inside a black hole, for example.

But quantum computers wont reach their real potential which will require harnessing the power of millions of qubits for more than a decade. Exactly what possibilities exist for the long-term future of quantum computers is still up in the air.

The outlook is similar to the patchy vision that surrounded the development of standard computers which quantum scientists refer to as classical computers in the middle of the 20th century. When they began to tinker with electronic computers, scientists couldnt fathom all of the eventual applications; they just knew the machines possessed great power. From that initial promise, classical computers have become indispensable in science and business, dominating daily life, with handheld smartphones becoming constant companions (SN: 4/1/17, p. 18).

Were very excited about the potential to really revolutionize what we can compute.

Krysta Svore

Since the 1980s, when the idea of a quantum computer first attracted interest, progress has come in fits and starts. Without the ability to create real quantum computers, the work remained theoretical, and it wasnt clear when or if quantum computations would be achievable. Now, with the small quantum computers at hand, and new developments coming swiftly, scientists and corporations are preparing for a new technology that finally seems within reach.

Companies are really paying attention, Microsofts Krysta Svore said March 13 in New Orleans during a packed session at a meeting of the American Physical Society. Enthusiastic physicists filled the room and huddled at the doorways, straining to hear as she spoke. Svore and her team are exploring what these nascent quantum computers might eventually be capable of. Were very excited about the potential to really revolutionize what we can compute.

Quantum computings promise is rooted in quantum mechanics, the counterintuitive physics that governs tiny entities such as atoms, electrons and molecules. The basic element of a quantum computer is the qubit (pronounced CUE-bit). Unlike a standard computer bit, which can take on a value of 0 or 1, a qubit can be 0, 1 or a combination of the two a sort of purgatory between 0 and 1 known as a quantum superposition. When a qubit is measured, theres some chance of getting 0 and some chance of getting 1. But before its measured, its both 0 and 1.

Because qubits can represent 0 and 1 simultaneously, they can encode a wealth of information. In computations, both possibilities 0 and 1 are operated on at the same time, allowing for a sort of parallel computation that speeds up solutions.

Another qubit quirk: Their properties can be intertwined through the quantum phenomenon of entanglement (SN: 4/29/17, p. 8). A measurement of one qubit in an entangled pair instantly reveals the value of its partner, even if they are far apart what Albert Einstein called spooky action at a distance.

Story continues after diagram

In quantum computing, programmers execute a series of operations, called gates, to flip qubits (represented by black horizontal lines), entangle them to link their properties, or put them in a superposition, representing 0 and 1 simultaneously. First, some gate definitions:

X gate: Flips a qubit from a 0 to a 1, or vice versa.

Hadamard gate: Puts a qubit into a superposition of states.

Controlled not gate: Flips a second qubit only if the first qubit is 1.

Scientists can combine gates like the ones above into complex sequences to perform calculations that are not possible with classical computers. One such quantum algorithm, called Grovers search, speeds up searches, such as scanning fingerprint databases for a match. To understand how this works, consider a simple game show.

In this game show, four doors hide one car and three goats. A contestant must open a door at random in hopes of finding the car. Grovers search looks at all possibilities at once and amplifies the desired one, so the contestant is more likely to find the car. The two qubits represent four doors, labeled in binary as 00, 01, 10 and 11. In this example, the car is hidden behind door 11.

Step 1:Puts both qubits in a superposition. All four doors have equal probability. Step 2:Hides the car behind door 11. In a real-world example, this information would be stored in a quantum database. Step 3:Amplifies the probability of getting the correct answer, 11, when the qubits are measured. Step 4: Measures both qubits; the result is 11.

Source: IBM Research; Graphics: T. Tibbitts

Such weird quantum properties can make for superefficient calculations. But the approach wont speed up solutions for every problem thrown at it. Quantum calculators are particularly suited to certain types of puzzles, the kind for which correct answers can be selected by a process called quantum interference. Through quantum interference, the correct answer is amplified while others are canceled out, like sets of ripples meeting one another in a lake, causing some peaks to become larger and others to disappear.

One of the most famous potential uses for quantum computers is breaking up large integers into their prime factors. For classical computers, this task is so difficult that credit card data and other sensitive information are secured via encryption based on factoring numbers. Eventually, a large enough quantum computer could break this type of encryption, factoring numbers that would take millions of years for a classical computer to crack.

Quantum computers also promise to speed up searches, using qubits to more efficiently pick out an information needle in a data haystack.

Qubits can be made using a variety of materials, including ions, silicon or superconductors, which conduct electricity without resistance. Unfortunately, none of these technologies allow for a computer that will fit easily on a desktop. Though the computer chips themselves are tiny, they depend on large cooling systems, vacuum chambers or other bulky equipment to maintain the delicate quantum properties of the qubits. Quantum computers will probably be confined to specialized laboratories for the foreseeable future, to be accessed remotely via the internet.

That vision of Web-connected quantum computers has already begun to Quantum computing is exciting. Its coming, and we want a lot more people to be well-versed in itmaterialize. In 2016, IBM unveiled the Quantum Experience, a quantum computer that anyone around the world can access online for free.

Quantum computing is exciting. Its coming, and we want a lot more people to be well-versed in it.

Jerry Chow

With only five qubits, the Quantum Experience is limited in what you can do, says Jerry Chow, who manages IBMs experimental quantum computing group. (IBMs 16-qubit computer is in beta testing, so Quantum Experience users are just beginning to get their hands on it.) Despite its limitations, the Quantum Experience has allowed scientists, computer programmers and the public to become familiar with programming quantum computers which follow different rules than standard computers and therefore require new ways of thinking about problems. Quantum computing is exciting. Its coming, and we want a lot more people to be well-versed in it, Chow says. Thatll make the development and the advancement even faster.

But to fully jump-start quantum computing, scientists will need to prove that their machines can outperform the best standard computers. This step is important to convince the community that youre building an actual quantum computer, says quantum physicist Simon Devitt of Macquarie University in Sydney. A demonstration of such quantum supremacy could come by the end of the year or in 2018, Devitt predicts.

Researchers from Google set out a strategy to demonstrate quantum supremacy, posted online at arXiv.org in 2016. They proposed an algorithm that, if run on a large enough quantum computer, would produce results that couldnt be replicated by the worlds most powerful supercomputers.

The method involves performing random operations on the qubits, and measuring the distribution of answers that are spit out. Getting the same distribution on a classical supercomputer would require simulating the complex inner workings of a quantum computer. Simulating a quantum computer with more than about 45 qubits becomes unmanageable. Supercomputers havent been able to reach these quantum wilds.

To enter this hinterland, Google, which has a nine-qubit computer, has aggressive plans to scale up to 49 qubits. Were pretty optimistic, says Googles John Martinis, also a physicist at the University of California, Santa Barbara.

Martinis and colleagues plan to proceed in stages, working out the kinks along the way. You build something, and then if its not working exquisitely well, then you dont do the next one you fix whats going on, he says. The researchers are currently developing quantum computers of 15 and 22 qubits.

IBM, like Google, also plans to go big. In March, the company announced it would build a 50-qubit computer in the next few years and make it available to businesses eager to be among the first adopters of the burgeoning technology. Just two months later, in May, IBM announced that its scientists had created the 16-qubit quantum computer, as well as a 17-qubit prototype that will be a technological jumping-off point for the companys future line of commercial computers.

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But a quantum computer is much more than the sum of its qubits. One of the real key aspects about scaling up is not simply qubit number, but really improving the device performance, Chow says. So IBM researchers are focusing on a standard they call quantum volume, which takes into account several factors. These include the number of qubits, how each qubit is connected to its neighbors, how quickly errors slip into calculations and how many operations can be performed at once. These are all factors that really give your quantum processor its power, Chow says.

Errors are a major obstacle to boosting quantum volume. With their delicate quantum properties, qubits can accumulate glitches with each operation. Qubits must resist these errors or calculations quickly become unreliable. Eventually, quantum computers with many qubits will be able to fix errors that crop up, through a procedure known as error correction. Still, to boost the complexity of calculations quantum computers can take on, qubit reliability will need to keep improving.

Different technologies for forming qubits have various strengths and weaknesses, which affect quantum volume. IBM and Google build their qubits out of superconducting materials, as do many academic scientists. In superconductors cooled to extremely low temperatures, electrons flow unimpeded. To fashion superconducting qubits, scientists form circuits in which current flows inside a loop of wire made of aluminum or another superconducting material.

Several teams of academic researchers create qubits from single ions, trapped in place and probed with lasers. Intel and others are working with qubits fabricated from tiny bits of silicon known as quantum dots (SN: 7/11/15, p. 22). Microsoft is studying what are known as topological qubits, which would be extra-resistant to errors creeping into calculations. Qubits can even be forged from diamond, using defects in the crystal that isolate a single electron. Photonic quantum computers, meanwhile, make calculations using particles of light. A Chinese-led team demonstrated in a paper published May 1 in Nature Photonics that a light-based quantum computer could outperform the earliest electronic computers on a particular problem.

One company, D-Wave, claims to have a quantum computer that can perform serious calculations, albeit using a more limited strategy than other quantum computers (SN: 7/26/14, p. 6). But many scientists are skeptical about the approach. The general consensus at the moment is that something quantum is happening, but its still very unclear what it is, says Devitt.

While superconducting qubits have received the most attention from giants like IBM and Google, underdogs taking different approaches could eventually pass these companies by. One potential upstart is Chris Monroe, who crafts ion-based quantum computers.

On a walkway near his office on the University of Maryland campus in College Park, a banner featuring a larger-than-life portrait of Monroe adorns a fence. The message: Monroes quantum computers are a fearless idea. The banner is part of an advertising campaign featuring several of the universitys researchers, but Monroe seems an apt choice, because his research bucks the trend of working with superconducting qubits.

Monroe and his small army of researchers arrange ions in neat lines, manipulating them with lasers. In a paper published in Nature in 2016, Monroe and colleagues debuted a five-qubit quantum computer, made of ytterbium ions, allowing scientists to carry out various quantum computations. A 32-ion computer is in the works, he says.

Monroes labs he has half a dozen of them on campus dont resemble anything normally associated with computers. Tables hold an indecipherable mess of lenses and mirrors, surrounding a vacuum chamber that houses the ions. As with IBMs computer, although the full package is bulky, the quantum part is minuscule: The chain of ions spans just hundredths of a millimeter.

Scientists in laser goggles tend to the whole setup. The foreign nature of the equipment explains why ion technology for quantum computing hasnt taken off yet, Monroe says. So he and colleagues took matters into their own hands, creating a start-up called IonQ, which plans to refine ion computers to make them easier to work with.

Monroe points out a few advantages of his technology. In particular, ions of the same type are identical. In other systems, tiny differences between qubits can muck up a quantum computers operations. As quantum computers scale up, Monroe says, there will be a big price to pay for those small differences. Having qubits that are identical, over millions of them, is going to be really important.

In a paper published in March in Proceedings of the National Academy of Sciences, Monroe and colleagues compared their quantum computer with IBMs Quantum Experience. The ion computer performed operations more slowly than IBMs superconducting one, but it benefited from being more interconnected each ion can be entangled with any other ion, whereas IBMs qubits can be entangled only with adjacent qubits. That interconnectedness means that calculations can be performed in fewer steps, helping to make up for the slower operation speed, and minimizing the opportunity for errors.

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Two different quantum computers one using ion qubits, the other superconducting qubits went head-to-head in a recent comparison. Both five-qubit computers performed similarly, but each had its own advantages: The superconducting computer was faster; the ion computer was more interconnected, needing fewer steps to perform calculations.

Source: N.M. Linkeet al/PNAS2017

Computers like Monroes are still far from unlocking the full power of quantum computing. To perform increasingly complex tasks, scientists will have to correct the errors that slip into calculations, fixing problems on the fly by spreading information out among many qubits. Unfortunately, such error correction multiplies the number of qubits required by a factor of 10, 100 or even thousands, depending on the quality of the qubits. Fully error-corrected quantum computers will require millions of qubits. Thats still a long way off.

So scientists are sketching out some simple problems that quantum computers could dig into without error correction. One of the most important early applications will be to study the chemistry of small molecules or simple reactions, by using quantum computers to simulate the quantum mechanics of chemical systems. In 2016, scientists from Google, Harvard University and other institutions performed such a quantum simulation of a hydrogen molecule. Hydrogen has already been simulated with classical computers with similar results, but more complex molecules could follow as quantum computers scale up.

Once error-corrected quantum computers appear, many quantum physicists have their eye on one chemistry problem in particular: making fertilizer. Though it seems an unlikely mission for quantum physicists, the task illustrates the game-changing potential of quantum computers.

The Haber-Bosch process, which is used to create nitrogen-rich fertilizers, is hugely energy intensive, demanding high temperatures and pressures. The process, essential for modern farming, consumes around 1 percent of the worlds energy supply. There may be a better way. Nitrogen-fixing bacteria easily extract nitrogen from the air, thanks to the enzyme nitrogenase. Quantum computers could help simulate this enzyme and reveal its properties, perhaps allowing scientists to design a catalyst to improve the nitrogen fixation reaction, make it more efficient, and save on the worlds energy, says Microsofts Svore. Thats the kind of thing we want to do on a quantum computer. And for that problem it looks like well need error correction.

Pinpointing applications that dont require error correction is difficult, and the possibilities are not fully mapped out. Its not because they dont exist; I think its because physicists are not the right people to be finding them, says Devitt, of Macquarie. Once the hardware is available, the thinking goes, computer scientists will come up with new ideas.

Thats why companies like IBM are pushing their quantum computers to users via the Web. A lot of these companies are realizing that they need people to start playing around with these things, Devitt says.

Quantum scientists are trekking into a new, uncharted realm of computation, bringing computer programmers along for the ride. The capabilities of these fledgling systems could reshape the way society uses computers.

Eventually, quantum computers may become part of the fabric of our technological society. Quantum computers could become integrated into a quantum internet, for example, which would be more secure than what exists today (SN: 10/15/16, p. 13).

Quantum computers and quantum communication effectively allow you to do things in a much more private way, says physicist Seth Lloyd of MIT, who envisions Web searches that not even the search engine can spy on.

There are probably plenty more uses for quantum computers that nobody has thought up yet.

Were not sure exactly what these are going to be used for. That makes it a little weird, Monroe says. But, he maintains, the computers will find their niches. Build it and they will come.

This story appears in the July 8, 2017, issue ofScience Newswith the headline, "Quantum Computers Get Real: As the first qubit-based machines come online, scientists are just beginning to imagine the possibilities."

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Quantum computers are about to get real - Science News Magazine

If you apply this idea to the structure of an atom, in the older, Bohr model, there is a nucleus and there are rings (levels) of energy around the nucleus. The length of each orbit was related to a wavelength. No two electrons can have all the same wave characteristics. Scientists now say that electrons behave like waves, and fill areas of the atom like sound waves might fill a room. The electrons, then, exist in something scientists call "electron clouds". The size of the shells now relates to the size of the cloud. This is where the spdf stuff comes in, as these describe the shape of the clouds.

Look at the Heisenberg uncertainty principle in a more general way using the observer effect. While Heisenberg looks at measurements, you can see parallels in larger observations. You can not observe something naturally without affecting it in some way. The light and photons used to watch an electron would move the electron. When you go out in a field in Africa and the animals see you, they will act differently. If you are a psychiatrist asking a patient some questions, you are affecting him, so the answers may be changed by the way the questions are worded. Field scientists work very hard to try and observe while interfering as little as possible.

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Quantum Physics News - Phys.org - News and Articles on ...

Intern Katherine Dunne with mentor Maurice Garcia-Sciveres. (Credit: Marilyn Chung/Berkeley Lab)

As a high school student in Birmingham, Alabama, Berkeley Lab Undergraduate Research (BLUR) intern Katie Dunne first dreamed of becoming a physicist after reading Albert Einsteins biography, but didnt know anyone who worked in science. I felt like the people who were good at math and science werent my friends, she said. So when it came time for college, she majored in English, and quickly grew dissatisfied because it wasnt challenging enough. Eventually, she got to know a few engineers, but none of them were women, she recalled.

She still kept physics in the back of her mind until she read an article about The First Lady of Physics, Chien-Shiung Wu, an experimental physicist who worked on the Manhattan Project, and later designed the Wu experiment, which proved that the conservation of parity is violated by weak interactions. Two male theorists who proposed parity violation won the 1957 Nobel Prize in physics, and Wu did not, Dunne said. When I read about her, I decided that thats what I want to do design experiments.

Katie Dunne, left, and mentor Maurice Garcia-Sciveres. (Credit: Marilyn Chung/Berkeley Lab)

So she put physics front and center, and about four years ago, transferred as a physics major to the City College of San Francisco. With Silicon Valley nearby, there are many opportunities here to get work experience in instrumentation and electrical engineering, Dunne said. In the summers of 2014 and 2015, she landed internships in the Human Factors division at NASA Ames Research Center in Mountain View, where she streamlined the development of a printed circuit board for active infrared illumination.

But it wasnt until she took a class in modern physics when she discovered her true passion particle physics. When we got to quantum physics, it was great. Working on the problems of quantum physics is exciting, she said. Its so elegant and dovetails with math. Its the ultimate mystery because we cant observe quantum behavior.

When it came time to apply for her next summer internship in 2016, instead of reapplying for a position at NASA, she googled ATLAS, the name of a 7,000-ton detector for the Large Hadron Collider (LHC). Her search drummed up an article about Beate Heinemann, who, at the time, was a researcher with dual appointments at UC Berkeley and Berkeley Lab and was deputy spokesperson of the ATLAS collaboration. (Heinemann is also one of the 20percent of female physicists working on the ATLAS experiment.)

When Dunne contacted Heinemann to ask if she would consider her for an internship, she suggested that she contact Maurice Garcia-Sciveres, a physicist at Berkeley Lab whose research specializes in pixel detectors for ATLAS, and who has mentored many students interested in instrumentation.

Garcia-Sciveres invited Dunne to a meeting so she could see the kind of work that they do. I could tell I would get a lot of hands-on experience, she said. So she applied for her first internship with Garcia-Sciveres through the Community College Internship (CCI) program which, like the BLUR internship program, is managed by Workforce Development & Education at Berkeley Lab and started to work with his team on building prototype integrated circuit (IC) test systems for ATLAS as part of the High Luminosity Large Hadron Collider (HL-LHC) Project, an international collaboration headed by CERN to increase the LHCs luminosity (rate of collisions) by a factor of 10 by 2020.

A quad module with a printed circuit board (PCB) for power and data interface to four FE-I4B chips. Dunne designed the PCB. (Credit: Katie Dunne/Berkeley Lab)

For the ATLAS experiment, we work with the Engineering Division to build custom electronics and integrated circuits for silicon detectors. Our work is focused on improving the operation, testing, and debugging of these ICs, said Garcia-Sciveres.

During Dunnes first internship, she analyzed threshold scans for an IC readout chip, and tested their radiation hardness or threshold for tolerating increasing radiation doses at the Labs 88-Inch Cyclotron and at SLAC National Accelerator Laboratory. Berkeley Lab is a unique environment for interns. They throw you in, and you learn on the job. The Lab gives students opportunities to make a difference in the field theyre working in, she said.For the ATLAS experiment, we work with the Engineering Division to build custom electronics and integrated circuits for silicon detectors. Our work is focused on improving the operation, testing, and debugging of these ICs, said Garcia-Sciveres.

For Garcia-Sciveres, it didnt take long for Dunne to prove she could make a difference for his team. Just after her first internship at Berkeley Lab, the results from her threshold analysis made their debut as data supporting his presentation at the 38th International Conference on High Energy Physics (ICHEP) in August 2016. The results were from her measurements, he said. This is grad student-level work shes been doing. Shes really good.

Katie Dunne delivers a poster presentation in spring 2017. (Credit: Marilyn Chung/Berkeley Lab)

After the conference, Garcia-Sciveres asked Dunne to write the now published proceedings (he and the other authors provided her with comments and suggested wording). And this past January, Dunne presented Results of FE65-P2 Stability Tests for the High Luminosity LHC Upgrade during the HL-LHC, BELLE2, Future Colliders session of the American Physical Society (APS) Meeting in Washington, D.C.

This summer, for her third and final internship at the Lab, Dunne is working on designing circuit boards needed for the ATLAS experiment, and assembling and testing prototype multi-chip modules to evaluate system performance. She hopes to continue working on ATLAS when she transfers to UC Santa Cruz as a physics major in the fall, and would like to get a Ph.D. in physics one day. I love knowing that the work I do matters. My internships and work experience as a research assistant at Berkeley Lab have made me more confident in the work Im doing, and more passionate about getting things done and sharing my results, she said.

Goherefor more information about internships hosted by Workforce Development & Education at Berkeley Lab, or contact them ateducation@lbl.gov.

This work was supported in part by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Community College Internship (CCI) program.

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Berkeley Lab Intern Finds Her Way in Particle Physics | Berkeley Lab - Lawrence Berkeley National Laboratory

Albert Einstein is heading out for his daily stroll and has to pass through two doorways. First he walks through the green door, and then through the red one. Or wait did he go through the red first and then the green? It must have been one or the other. The events had have to happened in a sequence, right?

Not if Einstein were riding on one of the photons ricocheting through Philip Walther's lab at the University of Vienna. Walther's group has shown that it is impossible to say in which order these photons pass through a pair of gates as they zip around the lab. It's not that this information gets lost or jumbled it simply doesn't exist. In Walther's experiments, there is no well-defined order of events.

This finding1 in 2015 made the quantum world seem even stranger than scientists had thought. Walther's experiments mash up causality: the idea that one thing leads to another. It is as if the physicists have scrambled the concept of time itself, so that it seems to run in two directions at once.

In everyday language, that sounds nonsensical. But within the mathematical formalism of quantum theory, ambiguity about causation emerges in a perfectly logical and consistent way. And by creating systems that lack a clear flow of cause and effect2, researchers now think they can tap into a rich realm of possibilities. Some suggest that they could boost the already phenomenal potential of quantum computing. A quantum computer free from the constraints of a predefined causal structure might solve some problems faster than conventional quantum computers, says quantum theorist Giulio Chiribella of the University of Hong Kong.

What's more, thinking about the 'causal structure' of quantum mechanics which events precede or succeed others might prove to be more productive, and ultimately more intuitive, than couching it in the typical mind-bending language that describes photons as being both waves and particles, or events as blurred by a haze of uncertainty.

And because causation is really about how objects influence one another across time and space, this new approach could provide the first steps towards uniting the two cornerstone theories of physics and resolving one of the most profound scientific challenges today. Causality lies at the interface between quantum mechanics and general relativity, says Walther's collaborator aslav Brukner, a theorist at the Institute for Quantum Optics and Quantum Information in Vienna, and so it could help us to think about how one could merge the two conceptually.

Causation has been a key issue in quantum mechanics since the mid-1930s, when Einstein challenged the apparent randomness that Niels Bohr and Werner Heisenberg had installed at the heart of the theory. Bohr and Heisenberg's Copenhagen interpretation insisted that the outcome of a quantum measurement such as checking the orientation of a photon's plane of polarization is determined at random, and only in the instant that the measurement is made. No reason can be adduced to explain that particular outcome. But in 1935, Einstein and his young colleagues Boris Podolsky and Nathan Rosen (now collectively denoted EPR) described a thought experiment that pushed Bohr's interpretation to a seemingly impossible conclusion.

The EPR experiment involves two particles, A and B, that have been prepared with interdependent, or 'entangled', properties. For example, if A has an upward-pointing 'spin' (crudely, a quantum property that can be pictured a little bit like the orientation of a bar magnet), then B must be down, and vice versa.

Both pairs of orientations are possible. But researchers can discover the actual orientation only when they make a measurement on one of the particles. According to the Copenhagen interpretation, that measurement doesn't just reveal the particle's state; it actually fixes it in that instant. That means it also instantly fixes the state of the particle's entangled partner however far away that partner is. But Einstein considered this apparent instant action at a distance impossible, because it would require faster-than-light interaction across space, which is forbidden by his special theory of relativity. Einstein was convinced that this invalidated the Copenhagen interpretation, and that particles A and B must already have well-defined spins before anybody looks at them.

Measurements of entangled particles show, however, that the observed correlation between the spins can't be explained on the basis of pre-existing properties. But these correlations don't actually violate relativity because they can't be used to communicate faster than light. Quite how the relationship arises is hard to explain in any intuitive cause-and-effect way.

But what the Copenhagen interpretation does at least seem to retain is a time-ordering logic: a measurement can't induce an effect until after it has been made. For event A to have any effect on event B, A has to happen first. The trouble is that this logic has unravelled over the past decade, as researchers have realized that it is possible to imagine quantum scenarios in which one simply can't say which of two related events happens first.

Classically, this situation sounds impossible. True, we might not actually know whether A or B happened first but one of them surely did. Quantum indeterminacy, however, isn't a lack of knowledge; it's a fundamental prohibition on pronouncing on any 'true state of affairs' before a measurement is made.

Brukner's group in Vienna, Chiribella's team and others have been pioneering efforts to explore this ambiguous causality in quantum mechanics3, 4. They have devised ways to create related events A and B such that no one can say whether A preceded and led to (in a sense 'caused') B, or vice versa. This arrangement enables information to be shared between A and B in ways that are ruled out if there is a definite causal order. In other words, an indeterminate causal order lets researchers do things with quantum systems that are otherwise impossible.

The trick they use involves creating a special type of quantum 'superposition'. Superpositions of quantum states are well known: a spin, for example, can be placed in a superposition of up and down states. And the two spins in the EPR experiment are in a superposition in that case involving two particles. It's often said that a quantum object in a superposition exists in two states at once, but more properly it simply cannot be said in advance what the outcome of a measurement would be. The two observable states can be used as the binary states (1 and 0) of quantum bits, or qubits, which are the basic elements of quantum computers.

The researchers extend this concept by creating a causal superposition. In this case, the two states represent sequences of events: a particle goes first through gate A and then through gate B (so that A's output state determines B's input), or vice versa.

In 2009, Chiribella and his co-workers came up with a theoretical way to do an experiment like this using a single qubit as a switch that controls the causal order of events experienced by a particle that acts as second qubit3. When the control-switch qubit is in state 0, the particle goes through gate A first, and then through gate B. When the control qubit is in state 1, the order of the second qubit is BA. But if that qubit is in a superposition of 0 and 1, the second qubit experiences a causal superposition of both sequences meaning there is no defined order to the particle's traversal of the gates (see 'Trippy journeys').

Nik Spencer/Nature

Three years later, Chiribella proposed an explicit experimental procedure for enacting this idea5; Walther, Brukner and their colleagues subsequently worked out how to implement it in the lab1. The Vienna team uses a series of 'waveplates' (crystals that change a photon's polarization) and partial mirrors that reflect light and also let some pass through. These devices act as the logic gates A and B to manipulate the polarization of a test photon. A control qubit determines whether the photon experiences AB or BA or a causal superposition of both. But any attempt to find out whether the photon goes through gate A or gate B first will destroy the superposition of gate ordering.

Having demonstrated causal indeterminacy experimentally, the Vienna team wanted to go further. It's one thing to create a quantum superposition of causal states, in which it is simply not determined what caused what (that is, whether the gate order is AB or BA). But the researchers wondered whether it is possible to preserve causal ambiguity even if they spy on the photon as it travels through various gates.

At face value, this would seem to violate the idea that sustaining a superposition depends on not trying to measure it. But researchers are now realizing that in quantum mechanics, it's not exactly what you do that matters, but what you know.

Last year, Walther and his colleagues devised a way to measure the photon as it passes through the two gates without immediately changing what they know about it6. They encode the result of the measurement in the photon itself, but do not read it out at the time. Because the photon goes through the whole circuit before it is detected and the measurement is revealed, that information can't be used to reconstruct the gate order. It's as if you asked someone to keep a record of how they feel during a trip and then relay the information to you later so that you can't deduce exactly when and where they were when they wrote it down.

As the Vienna researchers showed, this ignorance preserves the causal superposition. We don't extract any information about the measurement result until the very end of the entire process, when the final readout takes place, says Walther. So the outcome of the measurement process, and the time when it was made, are hidden but still affect the final result.

Other teams have also been creating experimental cases of causal ambiguity by using quantum optics. For example, a group at the University of Waterloo in Canada and the nearby Perimeter Institute for Theoretical Physics has created quantum circuits that manipulate photon states to produce a different causal mash-up. In effect, a photon passes through gates A and B in that order, but its state is determined by a mixture of two causal procedures: either the effect of B is determined by the effect of A, or the effects of A and B are individually determined by some other event acting on them both, in much the same way that a hot day can increase sunburn cases and ice-cream sales without the two phenomena being directly causally related. As with the Vienna experiments, the Waterloo group found that it's not possible to assign a single causal 'story' to the state the photons acquire7.

Some of these experiments are opening up new opportunities for transmitting information. A causal superposition in the order of signals travelling through two gates means that each can be considered to send information to the other simultaneously. Crudely speaking, you get two operations for the price of one, says Walther. This offers a potentially powerful shortcut for information processing.

An indeterminate causal order lets researchers do things with quantum systems that are otherwise impossible.

Although it has long been known that using quantum superposition and entanglement could exponentially increase the speed of computation, such tricks have previously been played only with classical causal structures. But the simultaneous nature of pathways in a quantum-causal superposition offers a further boost in speed. That potential was apparent when such superpositions were first proposed: quantum theorist Lucien Hardy at the Perimeter Institute8 and Chiribella and his co-workers3 independently suggested that quantum computers operating with an indefinite causal structure might be more powerful than ones in which causality is fixed.

Last year, Brukner and his co-workers showed9 that building such a shortcut into an information-processing protocol with many gates should give an exponential increase in the efficiency of communication between gates, which could be beneficial for computation. We haven't reached the end yet of the possible speed-ups, says Brukner. Quantum mechanics allows way more.

It's not terribly complicated to build the necessary quantum-circuit architectures, either you just need quantum switches similar to those Walther has used. I think this could find applications soon, Brukner says.

The bigger goal, however, is theoretical. Quantum causality might supply a point of entry to some of the hardest questions in physics such as where quantum mechanics comes from.

Quantum theory has always looked a little ad hoc. The Schrdinger equation works marvellously to predict the outcomes of quantum experiments, but researchers are still arguing about what it means, because it's not clear what the physics behind it is. Over the past two decades, some physicists and mathematicians, including Hardy10 and Brukner11, have sought to clarify things by building 'quantum reconstructions': attempts to derive at least some characteristic properties of quantum-mechanical systems such as entanglement and superpositions from simple axioms about, say, what can and can't be done with the information encoded in the states (see Nature 501, 154156; 2013).

The framework of causal models provides a new perspective on these questions, says Katja Ried, a physicist at the University of Innsbruck in Austria who previously worked with the University of Waterloo team on developing systems with causal ambiguity. If quantum theory is a theory about how nature processes and distributes information, then asking in which ways events can influence each other may reveal the rules of this processing.

And quantum causality might go even further by showing how one can start to fit quantum theory into the framework of general relativity, which accounts for gravitation. The fact that causal structure plays such a central role in general relativity motivates us to investigate in which ways it can 'behave quantumly', says Ried.

Most of the attempts to understand quantum mechanics involve trying to save some aspects of the old classical picture, such as particle trajectories, says Brukner. But history shows us that what is generally needed in such cases is something more, he says something that goes beyond the old ideas, such as a new way of thinking about causality itself. When you have a radical theory, to understand it you usually need something even more radical.

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